Optical sensors, including (ambient) light sensors, proximity sensors and color sensors, for example, are used in various applications to detect the presence of (ambient) light, motion or proximity of an object as well as light color and intensity. Besides light sensitive components such as photo-diodes optical sensors often comprise dedicated filters such as interference or plasmonic filters.
An interference filter, or dichroic filter, is an optical filter that reflects one or more spectral lines or spectral bands while transmitting others. Typically, these types of filters maintain very low absorption for all wavelengths of interest. Interference filters consists of alternating layers of dielectric materials and thin metal films with varying index of refraction in the ultra-violet, visible and/or infrared spectrum. The succession and properties of the layers can be controlled to a high degree and determine the optical properties of the interference filter, for example, whether it is a high-pass, low-pass, band-pass or band rejection filter.
Interference filters, however, have the inherent disadvantage that their optical properties are depending on the angle under which light incidents upon the filter. Among other things directional changes in the optical properties of an interference filter are affected by increasing the angle of incidence or angle of aperture. Generally, the transmission wavelength or edge position of interference filters is shifted towards shorter wavelengths when increasing the angle of incidence. FIG. 6 shows the transmission (in %) as a function of wavelength λ (in nm) for several transmission curves T1, T2, T3, and T4 of a tristimulus X1 filter, as an example. The filter is designed for parallel incident light. The transmission curves T1, T2, T3, T4 have been recorded for 30°, 20°, 10°, 0° angle of incidence (measured with respect to a surface normal of the filter), respectively. The shift of the filter characteristic in dependence on the angle of the incident, parallel light is considerable.
FIG. 7 shows transmission curves T5, T6, T7, and T8 of a tristimulus X2 filter which is designed for diffusive incident light having the same target function as the filter X1 of FIG. 5. Interference filters can be adapted for diffused incident light by means of succession and properties of the layers which make up the filter, e.g. layer distances and index of refraction. This can be achieved by dedicated optical simulation software, for example. The transmission curves T5, T6, T7, T8 have been recorded for 0°, 10°, 20°, 30° angle of incidence (measured with respect to a surface normal of the filter), respectively. The shift of the filter characteristic in dependence on the angle of the incident, diffusive light is largely reduced (compare also transmission curve T*). As can be seen in the drawing interference filters adapted for diffused incident light have practically insignificant shift of the filter characteristic angles of incidence for 0°, 10°, 20°, 30° angle of incidence, i.e. smaller than 20 to 10 nm, in particular smaller than 5 nm. However, such filter does not show a good performance under typical operating conditions without further external optics.
Recently, plasmonic colour filters have become available which employ surface-plasmon based nanostructures. Such plasmon filters offer the ability to efficiently manipulate light due to their small dimensions. By changing their nanostructure, colour filters can be produced which are capable of transmitting arbitrary colours at high spatial resolution with the promise of compact device architectures. However, the spectral responsivity of plasmonic filters also changes with the angle of incident light.
FIG. 5 shows a prior art solution of an optical sensor with an external aperture and external diffusor. Basically, a black cylindrical tube 26 is placed over the optical sensors 27. In order to avoid shadowing effects, it is advantageous to place an additional diffusor 28 on top of this cylinder. However, such a design is rather bulky and cannot be fully integrated, e.g. into a single and small sensor.
There is a need in the art to provide an integrated optical sensor and a method of producing an integrated optical sensor which supports wafer level integration of optical sensors with optical filters and reduced angular dependence of the filter transmission characteristics.